General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, NMR well logging includes generating a magnetic field within a borehole (e.g., under the earth's surface), applying a series of electromagnetic pulses to the area around the borehole, and measuring signals received in response to those pulses to determine characteristics of the area proximate to the borehole. Conventional characteristics of the area measured during NMR well logging include longitudinal (T1) and transverse (T2) relaxation times, as well as diffusion coefficients of the fluid inside the area. In addition to these one-dimensional (1D) measurements of relaxation times and diffusion coefficients, NMR logs can provide two-dimensional (2D) maps showing the correlation between diffusion and relaxation times (D-T2 or D-T1 maps) and the correlation between longitudinal and transverse relaxation times (T1-T2 maps). These measurements are typically used to determine rock properties such as porosity and permeability, as well as fluid properties such as the saturation of oil, water and gas. In some cases, these measurements are used to determine the viscosity of the oil. Often on these logs, particularly with 2D maps, the water, gas and oil signals can be distinguished, which aids in determining the saturations of the oil, gas and water. In addition, by looking at the position of the oil signal on the map, one can obtain an estimate of the viscosity of the oil, due to various correlations between log mean relaxation times and viscosity.
At present, conventional techniques known by the inventors do not use the D-T1, D-T2 or T1-T2 maps to estimate the mean size of the molecules in the oil, or other related properties of the oil. This is because no straightforward correlation exists between the molecule size in the mixture, and the diffusion coefficients and relaxation times. This is in contrast to viscosity, where the correlation between viscosity and relaxation times enables one to estimate the viscosity from the 1D and 2D maps. Because the diffusion coefficients and relaxation times for a molecule in a mixture depend upon properties of the other molecules in that mixture, it may not be possible to determine the size of that molecule of interest without knowledge about characteristics of the remaining mixture. Additionally, the diffusion coefficients and relaxation times are both pressure and temperature dependent, making it even more difficult to use this information to deduce molecule sizes in a mixture.